The combustion of natural gas fuels and oil in the combustor of a gas turbine power plant is known to produce undesirable levels of nitrogen oxide emissions. In order to reduce the level of NOx emissions, coolant, such as steam or water, is injected into the combustor. As is commonly known, steam or water which is injected into a combustor reduces the temperature of natural gas fuel and oil as it combusts and burns and, as a result, the combustion process produces less NOx.
Systems are known, such as that disclosed in Martens et al, U.S. Pat. No. 4,160,362, for controlling the flow of steam and water into a combustor in order to reduce the emissions of NOx in the gas turbine exhaust. Martens recognizes the problem that over-injection of steam or water, beyond that which is necessary to achieve a desired level of NOx emissions, results in an unnecessary increase in mass flow throughout the turbine and decreases the cycle efficiency. Therefore, it is desirable to properly limit the flow of coolant into the combustors, without sacrificing the necessary reductions in NOx emissions, in order to run the power plant at high efficiency.
Generally, sensors are located in the turbine exhaust stack for measuring the amount of NOx produced. It is desirable to use the output of the NOx sensor as a parameter for input into the control systems employed to control the flow of steam and water into the combustor. However, sensors are unreliable due to the fact that in some circumstances they completely fail to operate.
Thus, in order to account for the unreliability of the NOx sensors, steam or water flow into the combustor is scheduled as a function of the turbine load. This function, known as a standard load versus flow curve, is determined from emissions tests actually conducted on the operational unit, or one similar. During field testing, based on a predetermined NOx set point, the flow rate of steam or water which actually produces NOx emissions at the desired NOx set point is plotted as a function of the turbine load. Accordingly, during actual operating conditions, the steam flow set point which is necessary to produce a desired set point level of NOx emissions at a specific turbine load is determined from the standard load versus flow curve. The parameters from this curve can then be used in a system for controlling the flow of coolant into the combustor.
However, changes in environmental conditions, i.e. ambient temperature in the area of the combustor, as well as the turbine operating conditions, such as the position of the inlet guide vanes, affect this standard load versus flow curve. These variables influence the amount of steam flow which is actually necessary to produce a desired NOx emissions level. Since these variables are not taken into account when the standard load versus flow curve is generated, this curve has a certain amount of error built into it. In order to account for this error, the actual NOx level measured by the sensor is used as a parameter in the control system to adjust the standard curve. However, as discussed below, this adjustment may be limited due to the fact that sensors are known to fail completely under some conditions.
During actual operating conditions at a specific turbine load, in order to produce the desired set point level of NOx, the steam flow set point is determined from the standard load versus flow curve. At the start of the control cycle, valves inject steam or water into the turbine combustors at this set point flow rate. However, due to the error in the standard curve, the actual level of NOx produced and measured by the sensor will most likely vary from the NOx set point. Thus, the control system must account for this error such that the valves inject more or less steam, as compared to the steam flow set point, to bring the NOx level measured by the sensor to the NOx set point.
Devices are known which may be used in coolant injection control systems for measuring the error attributable to the variable conditions associated with the standard load versus flow curve. One such device is a summer as disclosed in Martens. The measured error is used to adjust the steam flow set point derived from the standard load versus flow curve, in order to account for the variables which affect that curve.
For example, FIG. 1 shows an adjusted standard load versus flow curve representative of that which would be produced using a summing device in accordance with the prior art. The dashed lines represent the adjusted steam flow set points. As can be seen, summing devices provide the error in a discrete amount, wherein the magnitude of the error is the same at all points on the curve. Thus, such a device provides for straight line bias of the control system.
However, a problem has been recognized in that the use of devices which provide for straight line bias control in coolant injection systems is inefficient in some circumstances and may damage the gas turbine. As shown in FIG. 1, where a control system employs straight line bias control devices, the amount of error which the system measures, and thus the range of error within which the system operates, is the same When the turbine is at low load as it is when it is at high load. Thus, where the measured NOx level exceeds the NOx set point, the adjustment above the steam flow set point in order to account for the error will be the same, whether operating at high load or low turbine load. At high turbine load, the additional amount of steam flow may be only a small percentage increase above the steam flow set point. However, at low turbine loads, the additional amount of steam flow may be a large percentage increase above the relatively low steam flow set point. Where the control system responds to such an increase at low load conditions, the valves inject coolant at a level which may be beyond that which is necessary to reduce the amount of NOx emissions, thus reducing the turbine cycle efficiency. It is also possible that the over-injection of coolant may result in flame-out of the combustor, resulting in malfunctioning of the turbine and possible damage.
The problem created by the use of straight line bias control devices in the fluid injection system is compounded in the case where more than one gas turbine is connected to a single emissions stack, where the stack has only one NOx sensor for the plurality of gas turbines. For example, where two gas turbines are connected to a single stack, during start-up conditions where it is common for both turbines to be on line with only one turbine having steam injection at the time, the unit with steam injection in operation will be adjusting its steam flow based upon a combined NOx level from both units and the possibility of over-injection is increased. Also, where two gas turbines are connected to a single stack and both are on line, but each is operating at a different load, straight line bias of the control system, using a summation of the control variables, results in fighting between the units.
Therefore, there is a need for a system for automatic control of the flow of coolant into the combustors of a gas turbine power plant in order to reduce NOx emissions levels, wherein the control system employs devices which provide for a percentage bias of the system parameters. The present invention provides a system which satisfies this need.